Smoke characteristics of conveyor-belt fires in U-shaped belt-conveyor roadways under different ventilation velocities

To elucidate the smoke characteristics of conveyor-belt fires in U-shaped belt-conveyor roadways under different ventilation velocities, small-scale combustion experiments were conducted using PVG1250S flame-retardant conveyor-belt specimens. A PyroSim model of a U-shaped roadway was subsequently developed to examine smoke transport under different ventilation velocities and prescribed thermal-source intensities. The results show that, as the ventilation velocity increased from 0.4 to 1.6 m/s, the peak CO concentration at the monitoring point decreased from 453 to 175 ppm, whereas the peak flow-rate-corrected CO transport index increased by 54.43%. The fixed-point temperature response was non-monotonic and reflected the combined effects of combustion fluctuation, smoke-layer position, buoyancy, and convective transport. Numerical simulations further reveal that buoyancy dominates smoke movement under low-velocity ventilation, promoting upstream back-layering of hot smoke along the roof and local accumulation of heat and CO near the fire source. As the ventilation velocity increases, the longitudinal airflow increasingly suppresses and entrains the fire plume, thereby weakening upstream back-layering while enhancing smoke transport toward the downstream roadway, working face and connecting roadway. Based on the composite smoke-response index, the scenario-specific characteristic transition velocity associated with a marked reduction in long-distance upstream smoke back-layering was approximately 2.24 m/s, with a transition range of 2.0–2.4 m/s under the prescribed 4800 kW thermal-source sensitivity scenario. The results provide a reference for understanding smoke redistribution and ventilation effects in U-shaped belt-conveyor roadways under comparable source, geometric, and ventilation conditions.